Earlier this month, heavy thunderstorms marked the end of a heatwave. The storms caused damage around Winterswijk, where many trees were blown down by strong gusts of wind. The timing of these storms was no coincidence: heavy thunderstorms often occur in summer during or shortly after a warm period. Heat influences the ingredients necessary for a heavy rain shower.
How does a shower form?
To understand how showers are related to heat, it is important to first look at how a rain cloud forms. Three ingredients are needed: moisture, an unstable atmosphere, and something that causes air to rise. This last one can be a rain front, a hill that air moves against, or sufficient warmth at the ground.
As the air rises, it gradually cools. The relative humidity in the air increases until the water vapor in the air condenses into cloud droplets. This releases heat, making the air even warmer and lighter, enhancing the upward movement (image 1). The result is a cloud with high tops, where the upper cloud droplets freeze into ice particles. This can be seen as the edges of the cloud appear somewhat blurred at the top (image 2). The cloud droplets clump together and grow into raindrops. If they are large enough, they fall from the cloud as rain.
Within such a cloud, many movements take place: on one side, the rising movement of the relatively warm air, and on the other side, the falling movement of the cooled air from higher altitudes and raindrops. These air currents move alongside each other and also carry electrically charged particles. This creates an electrical charge. If the charge is large enough, a discharge occurs in the form of lightning.